2,646 research outputs found

### Quantum Simulations of Relativistic Quantum Physics in Circuit QED

We present a scheme for simulating relativistic quantum physics in circuit
quantum electrodynamics. By using three classical microwave drives, we show
that a superconducting qubit strongly-coupled to a resonator field mode can be
used to simulate the dynamics of the Dirac equation and Klein paradox in all
regimes. Using the same setup we also propose the implementation of the
Foldy-Wouthuysen canonical transformation, after which the time derivative of
the position operator becomes a constant of the motion.Comment: 13 pages, 3 figure

### Optimal path for a quantum teleportation protocol in entangled networks

Bellman's optimality principle has been of enormous importance in the
development of whole branches of applied mathematics, computer science, optimal
control theory, economics, decision making, and classical physics. Examples are
numerous: dynamic programming, Markov chains, stochastic dynamics, calculus of
variations, and the brachistochrone problem. Here we show that Bellman's
optimality principle is violated in a teleportation problem on a quantum
network. This implies that finding the optimal fidelity route for teleporting a
quantum state between two distant nodes on a quantum network with bi-partite
entanglement will be a tough problem and will require further investigation.Comment: 4 pages, 1 figure, RevTeX

### The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342

We provide an in-depth analysis of the He II and H I absorption in the
intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using
spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following
up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here
on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio
eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers
(soft radiation fields) with voids in the (H I) Ly-alpha distribution. These
effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc,
together with radiative transfer through a filamentary IGM whose opacity
variations control the penetration of 1-5 ryd radiation over 30-40 Mpc
distances. Owing to photon statistics and backgrounds, we can measure optical
depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and
reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to
460. Values of eta = 20-200 are consistent with models of photoionization by
quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may
require additional contributions from starburst galaxies, heavily filtered
quasar radiation, or density variations. Regions with eta < 30 may indicate the
presence of local hard sources. We find that eta is higher in "void" regions,
where H I is weak or undetected and 80% of the path length has eta > 100. These
voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation
softened by escape from the AGN cores or transfer through the "cosmic web". The
apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag
between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.Comment: 27 pages, 7 figures, to appear in Ap

### Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols

We show that there exists a gap between the performance of separable and
collective measurements in qubit mixed-state estimation that persists in the
large sample limit. We characterize such gap in terms of the corresponding
bounds on the mean fidelity. We present an adaptive protocol that attains the
separable-measurement bound. This (optimal separable) protocol uses von Neumann
measurements and can be easily implemented with current technology.Comment: version published in PR

### Automated data reduction workflows for astronomy

Data from complex modern astronomical instruments often consist of a large
number of different science and calibration files, and their reduction requires
a variety of software tools. The execution chain of the tools represents a
complex workflow that needs to be tuned and supervised, often by individual
researchers that are not necessarily experts for any specific instrument. The
efficiency of data reduction can be improved by using automatic workflows to
organise data and execute the sequence of data reduction steps. To realize such
efficiency gains, we designed a system that allows intuitive representation,
execution and modification of the data reduction workflow, and has facilities
for inspection and interaction with the data. The European Southern Observatory
(ESO) has developed Reflex, an environment to automate data reduction
workflows. Reflex is implemented as a package of customized components for the
Kepler workflow engine. Kepler provides the graphical user interface to create
an executable flowchart-like representation of the data reduction process. Key
features of Reflex are a rule-based data organiser, infrastructure to re-use
results, thorough book-keeping, data progeny tracking, interactive user
interfaces, and a novel concept to exploit information created during data
organisation for the workflow execution. Reflex includes novel concepts to
increase the efficiency of astronomical data processing. While Reflex is a
specific implementation of astronomical scientific workflows within the Kepler
workflow engine, the overall design choices and methods can also be applied to
other environments for running automated science workflows.Comment: 12 pages, 7 figure

### Io: IUE observations of its atmosphere and the plasma torus

Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent

### On the p-length of some finite p-soluble groups

The main aim of this paper is to give structural information of a finite group of minimal order belonging to a subgroup-closed class of finite groups and whose p-length is greater than 1, p a prime number. Alternative proofs and improvements of recent results about the influence of minimal p-subgroups on the p-nilpotence and p-length of a finite group arise as consequences of our study

### Estimating the spectrum of a density matrix with LOCC

The problem of estimating the spectrum of a density matrix is considered.
Other problems, such as bipartite pure state entanglement, can be reduced to
spectrum estimation. A local operations and classical communication (LOCC)
measurement strategy is shown which is asymptotically optimal. This means that,
for a very large number of copies, it becomes unnecessary to perform collective
measurements which should be more difficult to implement in practice.Comment: 12 pages, uses iopart.cls and iopart10.clo. Improved version. v3:
Reference updated, added journal referenc

- …